raid1: use an int as the return value of raise_barrier()

Using a sector_t as the return value is misleading, because
raise_barrier() only return 0 or -EINTR.

Also add comments for the return values of raise_barrier().

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Song Liu <songliubraving@fb.com>